"A" and "B" open a channel and engage in several transactions. If immidiately after the most recent transaction, "transaction n", "B" losses all data associated with his LN wallet, and was unable to backup his data after "transaction n", then he will have insufficient information available to close the channel without risking all of his funds (with near certainty of loss). Remember that "B" does not have the current state of the channel.
If "B" tries to close the channel, he must do so using an old state, and attempting to use this would result in "A" being able to claim all funds in the channel.
The Lightning Network nodes are local and not on the blockchain so you can't restore backups deterministically. So without a backup you're SOL.
However in the future you could outsource the watching of commitment transactions to a Watchtower so if your peer tries to cheat you even if you're offline or lose your backup, then the Watchtower could post the penalty transaction on your behalf and take a cut of it as fees.
It's still very nascent at this stage though.
The Keypool feature was introduced in version 0.3.13.3 by Satoshi after a user lost a large amount of bitcoins because he didn't backup after his last transaction so he didn't have access to the private key of his change address.
In hindsight, using Keypool, HD, and mnemonics seem obvious to us. It wasn't obvious then.
On the future when LN has drastically improved, the improvements would seem obvious I'm hindsight too.